tracing/histogram: Fix documentation inline emphasis warning

This fixes the warning:

Documentation/trace/histogram.rst:1766: WARNING: Inline emphasis
start-string without end-string

The issue was caused by an unescaped '*' character.

Link: https://lore.kernel.org/all/20211028170548.2597449-1-kaleshsingh@google.com/T/#m77da47432f5cc6521d4294ffdb9621949cc35d04
Link: https://lkml.kernel.org/r/20211028170548.2597449-1-kaleshsingh@google.com

Fixes: 2d2f6d4b ("tracing/histogram: Document expression arithmetic and constants")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Documentation/trace/histogram.rst

@@ -1764,7 +1764,7 @@ using the same key and variable from yet another event::
   # echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger
 
 Expressions support the use of addition, subtraction, multiplication and
-division operators (+-*/).
+division operators (+-\*/).
 
 Note that division by zero always returns -1.